The present invention relates to a method of manufacturing a high-gloss, high-bulk paper product using a coating material comprising a particulate plastic pigment. The method of the invention permits calendering a base stock coated with such a coating material at lower roll temperatures than used in conventional gloss or soft calenders, and at lower nip loads than used in conventional supercalenders. As a result, densification of the resulting coated paper product is reduced, and a product having high gloss is obtained.
In papermaking, the finishing operation may be a calendering process, in which a paper web is passed between the nips formed between one or more pairs of rolls and the surface of the web is thereby flattened to form a smooth surface. Simultaneously, the thickness, or caliper, of the paper web is reduced and the web is densified. The density of the resulting product is usually calculated as:
Density=Basis Weight/Caliper
where the basis weight is the weight of a ream, in pounds, and the caliper is the thickness of the web, measured in thousandths of an inch, or points. Calendering generally reduces caliper, and, as a result, a higher density is obtained in the finished paper product. Bulk is inversely related to density, therefore when the density is increased, the bulk of the finished paper product will be reduced.
Calendering may generally be accomplished using a gloss calender, soft calender or supercalender. The gloss calender is typically comprised of a hard, non-resilient, heated roll made, for example, of steel, positioned proximally to a soft roll so as to form a narrow gap or nip. As the web passes through the nip it is exposed to a nip load in the range of from about 100 to about 900 pounds per lineal inch (pli). Nip pressures in this type of device are usually in the range of less than about 2000 pounds per square inch (psi). A wide range of processing temperatures can be used in a gloss calender, with the typical maximum temperature being in the range of about 450xc2x0 F. U.S. Pat. No. 5,118,533, for example, discloses a gloss calender in which the metal roll is held at a temperature of about 100-500xc2x0 C. (212-932xc2x0 F.). This temperature produces a high gloss finish on the surface of the web as it is passed through the nip, while the lower pressure used in a gloss calender causes less densification of the web, in comparison to a conventional supercalender.
The finishing effect achieved using the gloss calender, however, is not as smooth or as flat, and therefore not as glossy, as the surface produced using an apparatus capable of applying higher pressure. It is therefore often useful to increase the nip load or the roll temperature, or both, to plasticize and smooth the surface layers of the paper. Such modifications are incorporated, for example, in the design and operation of the conventional soft calender. The soft calender is usually constructed as having one to two nips per coated side, or as a two- or four-nip device, with each nip being formed between a heated hard roll and an unheated soft roll.
Alternatively, supercalendering may be used as the finishing operation. In such a process, the web is sequentially passed between a series of nips formed between the vertically stacked rolls of a supercalender. The supercalender typically comprises a frame having an upper roll and a lower roll between which are positioned intermediate rolls. The rolls of the supercalender may be heated hard rolls or unheated soft rolls, in serial or alternating arrangement. The nips formed between the rolls are typically shorter than those of a soft calender or gloss calender. The maximum temperature of the heated rolls in the supercalender is usually up to about 250xc2x0 F. As the web is passed through each nip, the web is compacted to form paper of substantially uniform density and high gloss by virtue of the repeated pressurization and heat exposure. The high pressure however also causes a reduction in bulk. In a supercalender, the nips are loaded initially by gravity, i.e., gravitational forces acting on the weight of the rolls themselves produce a distribution of the weight from the upper nip to the bottom nip that is substantially linear and increasing. This has the consequence that the load present in the bottom nip actually determines the minimum loading capacity of the calender.
Some paper and paperboard grades are sold by area, and, accordingly, a lower density sheet will give more surface area per ton of paper, which is advantageous for both the manufacturer and the end user. Thus it will be appreciated that a manufacturing method that will provide the desired surface finish on the base stock without substantially affecting its bulk is desirable. Where it is desirable to maintain more bulk in the finished product, using a conventional supercalender has typically been a disadvantage because such a process requires relatively high initial nip loads and corresponding nip pressures, which are at least maintained and, more often, increased as the web moves through the series of rolls. In this regard, a typical 10-12 roll supercalender device will produce a minimum load on the bottom nip in excess of about 1000 pli which could translate to a nip pressure greater than about 2500 psi depending upon the nip width. Moreover, in order to achieve some calendering potential from the upper nips, additional external load must be applied. For example, where the initial nip load may be about 1000 pounds per linear inch (pli) as the web enters the first nip, it is then exposed to subsequent nip loads at each of the successive intervening nips before passing through a final nip at a cumulative nip load of about 2000-3000 pli, which reflects the mass of each of the preceding rolls. As a result of this amount of pressurization in combination with heat, the web is highly densified to form a paper product having a high gloss surface. Since the pressure created by the extra loading at the nip is an important factor in achieving high gloss and smoothness, the result is a good finish for the web, but at the expense of an increase in density and loss of bulk.
In high throughput finishing operations, it is generally more efficient to use a supercalender to achieve the desired high gloss effect. However, as mentioned above, using the conventional supercalender exposes the base stock to linearly rising nip loads that may result in a glossy but highly densified product of reduced bulk. For example, U.S. Pat. No. 4,624,744 discloses a process that involves finishing a paper web at a nip pressure of at least about 2000 psi using a smooth metal finishing roll and a resilient backing roll, wherein the metal finishing roll is heated to a temperature sufficient to mold the web beneath its surface, generally referred to as substrata thermal molding. A comparison between supercalendering and gloss calendering is reported in the article entitled xe2x80x9cSupercalendering and Soft Nip Calendering Comparedxe2x80x9d, by John D. Peel, TAPPI Journal, October 1991, pp. 179-186.
A recent development in the calendering art addresses the problem of increasing linear loads at the successive nips in a supercalender. U.S. Pat. No. 5,438,920 describes a modified calender which is comprised of a series of rolls similar to a conventional supercalender, but in which the loading at each nip can be controlled by way of relief means that partially or completely relieve the nip loads produced by the masses of the intermediate rolls. In this regard, as the web passes through this calender, there is less variation in the nip load and nip pressure that is applied at each nip. As a result, there is less reduction in the bulk of the finished paper. This patent does not, however, teach or suggest making a high gloss paper of reduced bulk. Laid-open Canadian Patent Application 2238466AA, filed Dec. 20, 1998, teaches using another type of modified calender with reduced nip loads at each nip to make an ultra-light weight coated (ULWC) paper, which is a high-bulk glossed paper.
It is known in the papermaking art that various coating formulations and coating ingredients may be used in the manufacture of paper to achieve high gloss. For example, U.S. Pat. No. 5,283,129 discloses a lightweight paper stock that is coated with a pigment composition including delaminated clay, calcined clay and titanium dioxide, wherein up to about 5 parts by weight of hollow core opacifying plastic pigment may be substituted for the titanium dioxide. U.S. Pat. No. 4,010,307 discloses a high gloss coated paper product comprising 70-95% calcium carbonate and from 5-30% by weight of a non-film forming polymeric pigment having particles sized within the range of from 0.05-0.30 microns. U.S. Pat. No. 5,360,657 discloses a high gloss paper prepared by a rocess wherein a thermoplastic polymeric latex having a second order transition temperature of at least about 80xc2x0 C. (176xc2x0 F.), and an average particle size smaller than 100 microns, is applied to paper that is subsequently calendered. Laid-open Canadian Patent Application CA 2238466AA describes the manufacture of an ultra light weight (ULWC) paper by applying a coating containing 4 or more parts per 100 parts of a plastic coating pigment onto a base paper containing 60% weight or more mechanical pulp. The coated paper is calendered at a nip loading less than conventional supercalendering nip loading, to produce a product having a bulk factor above 51 if a supercalender is used, and a bulk factor above 60 if a hot-soft calender is used. The maximum TAPPI 75xc2x0 gloss achieved for ULWC paper using the invention of CA 2238466AA was reported as 35, while the inventors reported producing lightweight coated paper of lesser bulk having a maximum gloss value of 45. PCT published application WO 98/20201 discloses that a printing paper having high brightness and gloss can be manufactured by applying to paper a coating comprising at least 80 parts precipitated calcium carbonate and at least 5 parts of an acrylic styrene copolymer hollow sphere plastic pigment, based on 100 parts total weight of pigment, before finishing the coated paper to achieve gloss development. The finishing process does not involve using a modified supercalender, and the resulting paper is not taught as being a high bulk product. Hollow sphere pigments have also been used to produce a non-gloss finish. U.S. Pat. No. 5,902,453 teaches applying a coating containing 30-60% weight hollow sphere particle pigments and 40-70% weight cationic starch binder to a web, then calendering, under unspecified conditions, to yield a product with an uncoated appearance rather than a gloss finish. In an article entitled xe2x80x9cLightweight Coated Magazine Papers,xe2x80x9d published in the Jul. 5, 1976 issue of the magazine PAPER, Vol. 186, No. 1, at pages 35-38, a relationship between calendering and the use of plastic pigments in coatings is disclosed. The article notes, for example, that polymers such as polystyrene are thermoplastic and pressure sensitive, and a pigment based on polystyrene will exhibit a high degree of calendering response. Other publications, including the articles entitled xe2x80x9cLight Reflectance of Spherical Pigments in Paper Coatings,xe2x80x9d by J. Borch and P. Lepoutre, published in TAPPI, February 1978, Vol. 61, No. 2, at pages 45-48; xe2x80x9cPlastic Pigments in Paper Coatings,xe2x80x9d by B. Aluice and P. Lepoutre, published in TAPPI, May 1980, Vol. 63, No. 5, at pages 49-53; xe2x80x9cHollow-Sphere Polymer Pigment in Paper Coating,xe2x80x9d by J. E. Young, published in TAPPI, May 1985, Vol. 68, No. 5, at pages 102-105, all recognize the use of polymer pigments in paper coatings.
The foregoing references disclose making paper using polymeric coatings where the finishing means is a conventional calendering process, wherein high heat and/or high pressure are needed to produce a high gloss product; or, alternatively, as in CA application 2238466AA, a high bulk ULWC product of relatively low gloss is produced. A need exists, however, for a method of manufacturing paper or paperboard products in a supercalendering operation that reduces the loss of bulk and at the same time provides the finished product with a high gloss surface.
The need apparent in the art is met by the present invention, which provides a finishing method for paper and paperboard products that maintains bulk, to be used in combination with a coating formulation that provides high gloss and a smooth surface. The resulting calendered product is a high gloss paper or paperboard having an increased bulk-to-weight ratio.
In particular, the present invention relates to a method of producing a finished paper or paperboard having high gloss and high bulk, comprising:
a) forming a base stock;
b) applying to at least one side of the base stock a first layer of a coating formulation comprising a particulate plastic pigment to form a coated base stock; and
c) passing the coated base stock through the nips of a multi-nip calender device; wherein said calender device is comprised of one or more hard rolls and one or more soft rolls in linear arrangement, the interface between each pair of rolls forming a nip; wherein said calender device maintains a nip load at the initial nip of about 1000 pli or less, a nip load at each of the intervening nips of up about 1000 pli or less; and a nip load at the final nip of about 1000 pli or less; and wherein the surface temperature of the one or more hard rolls does not exceed about 450xc2x0 F.;
to form a product having a TAPPI 75xc2x0 gloss value of greater than 60 and a density of from about 15.5 pounds per ream per caliper point (lbs./ream/pt) to about 20 lbs./ream/pt, which corresponds inversely to a basis weight of about 50 lbs./ream to about 150 lbs./ream.
In another aspect, the invention relates to a high gloss paper product comprised of a paper base stock and a coating that includes a vacuolated particulate plastic pigment, having a density of from about 15.5 lbs./ream/pt to about 20 lbs./ream/pt, which corresponds inversely to a basis weight of from about 50 to about 150 lbs./ream, wherein the ream size is approximately 3300 ft2. Such densities are approximately 1-2 lbs./ream/pt lower than is typically accomplished using the same base stock material under conventional calendering methods at constant gloss levels. The products of the invention typically demonstrate a TAPPI 75xc2x0 gloss value of greater than about 60.